1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus, and more specifically, to a lithographic projection apparatus including radiation energy measurement, radiation absorber control, or both.
2. Description of the Related Art
The term “patterning structure” as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning structure is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning structure is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning structure is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the beam of radiation in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is desirable to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
In a lithographic projection apparatus it is desirable to ensure that the transmissivity of the radiation system and the projection system for radiation of the beam is substantially stable during an exposure of a target portion. This facilitates appropriate control over dose (“dose control”). Dose is defined as the total energy per unit area delivered to the substrate during an exposure of a target portion. Preferably, the transmissivity shall be substantially stable during a plurality of exposures of adjacent target portions such as to avoid a necessity of intermediate dose calibrations. It is known that instability of transmissivity can occur due to, for example, an interaction between the beam of radiation and materials of optical elements of the radiation and projection system. Known transient variations in the transmissivity of the optical elements can be corrected for via a feed-forward control. See, for example, U.S. Pat. No. 6,455,862.
For shorter wavelength radiation, especially for wavelength of 170 nm and below, absorption by air (through the presence of oxygen) becomes significant. Therefore, the optical path of the lithographic apparatus is evacuated or flushed (“purged”) with a gas (a “purge gas”) transparent to the radiation used, commonly dry N2. In spite of the above known precautions, there is the problem of significant transmissivity variations due to, for example, the presence of residual oxygen in the optical path, and leading to undesirable production errors. Evacuating the optical path to a high degree may reduce these causes of transmissivity losses even further, but transmissivity variations due to, for example, contamination or degradation of optical elements may still pose a problem. This is especially true at wavelengths of 20 nm and below.
At any wavelength, localized transmissivity changes may occur that may not be detectable by a dose control system. These localized changes may affect the energy of a region of the beam of radiation and consequently may cause incorrect imaging of a corresponding region on the target portion of the substrate. It is known in the art that these uniformity deviations may be corrected using a so-called “gray-filter”, which reduces the radiation in the regions with the highest energy down to the radiation in the regions of lowest energy. Refractive gray-filters may be manufactured, for example, by evaporating an appropriate metal onto specific regions of a transparent substrate. The gray-filter can be placed at any point in the optical path of the beam of radiation, but it is typically placed in the unpatterned beam of radiation between the source and the patterning device. This does not mean, however, that only uniformity deviations between the source and the patterning structure can be corrected—the fact that the measurement of the energy uniformity is typically done at substrate level means that the resulting gray-filter will create a uniform energy profile at substrate level independent of the position in the optical path where the transmissivity change has occurred. Gray-filters have the disadvantage, however, that they are manufactured based upon a single measurement, which means that they cannot respond dynamically to transmissivity changes or to changes in the operating mode.
A relatively inexpensive (for example, compared to using an expensive and bulky CCD camera) way of measuring the radiation uniformity is to use a photo-chromic material, such as UV SensorCards supplied by SensorPhysics (www.sensorphysics.com). These SensorCards are coated with a thin layer of polymer that changes its optical density upon exposure to photons to a degree that is proportional to the exposure intensity. The color change is instant and irreversible and no processing of the photo-chromatic material is required. Such polymers are used routinely with the ultraviolet (UV) and deep ultra violet (DUV) lithographic projection systems for measuring the energy profile in any cross-section of the beam of radiation (e.g. at intermediate focus, reticle, pupils, wafer or field facets) because it will give a footprint of the beam with very high resolution. An exposed SensorCard can be further read out with a standard computer scanner and processed with imaging software to increase the reproducibility of the measurement. It has been recently demonstrated that these polymers also respond to the wavelengths 5-20 nm (EUV) with a reasonable linearity and dynamic range to allow accurate measurements to be performed. However, these SensorCards can only be used once, and the disruption they cause to the radiation profile means that they cannot be left in the apparatus during normal operation.